Wearable potentiometric ion sensors

New trends in potentiometric sensors: From design to clinical and biomedical applications

Potentiometry, a well-established electrochemical technique, provides a powerful and versatile method for the sensitive and selective measurement of a variety of analytes by measuring the potential difference between two electrodes, allowing for a direct and rapid readout of ion concentrations. This makes it a valuable tool in a variety of applications including industry, agriculture, forensics, medical, environmental assessment, and pharmaceutical drug analysis, therefore it has received significant attention from the scientific community. Their broad implementation in sensing applications arises through their many benefits, including ease of design, fabrication, and modification; rapid response time; high selectivity; suitability for use with colored and/or turbid solutions; and potential for integration into embedded systems interfaces. Owing to these advantages and diverse applicability, sustained research and development in the field has resulted in the emergence of several notable trends in the field. 3D printing is the most recent technique used in potentiometry which offers many benefits such as improved flexibility and precision in the manufacturing of ion-selective electrodes and rapid prototyping decreases the time needed during optimization of important electrochemical parameters. Additionally, paper-based sensors are cost-effective and versatile platforms for in-field (point-of-care, POC) analysis, permitting rapid determination of a variety of analytes. One of the most interesting applications of potentiometry are wearable sensors which allow for the continuous monitoring of biomarkers, electrolytes and even pharmaceuticals, especially those with a narrow therapeutic index. Herein this review, we discuss several recent trends in potentiometric sensors since 2010, including 3D printing, paper-based devices, and other emerging techniques and the translation of potentiometric systems to wearable devices for the determination of ionic species or pharmaceuticals in biological fluids paving the way to various clinical and biomedical uses.

Colorimetric Detection of Potassium Ions by Electrochromic Thin Film Devices

Potassium ion (K+) detection technology based on electrochemical sensing has proven to be feasible and widely used in physiological real-time monitoring and pathological prediction. Nevertheless, chip integration, wireless communication needs, and specific detection limits present challenges. In this study, high-performance electrochromic materials (W18O49 nanowires) and ion-selective thin films are introduced to present a highly selective, colorimetric K+ biosensor, which demonstrates excellent selectivity in complex ionic environments. The thin film electrochromic electrode offers both colorimetric and electrochemical K+ detection simultaneously from 1 mm to 1000 mm concentration range, achieving up to ≈48% optical transmittance modulation, with a coloration time of 15 s. At the clinically relevant K+ concentration range of 1-10 mm, the electrode displays a linear transmittance modulation sensitivity of ≈0.6% per mm concentration change. Stability tests demonstrate that the colorimetric detection maintains 96% of its efficiency after 100 full concentration change cycles. A compact solid-state sensing device incorporating the electrochromic thin films has also been fabricated, and optically quantified K+ concentration in real-life sweat samples.

Solution-processable all-solid-state chloride-selective electrode: Enhanced sensitivity from anion dopant exchange

Ion-selective electrodes (ISEs) are widely used in many industries, with recent research focusing on their miniaturization by replacing the liquid filling solutions with solid-contacts. Solution-processing is the preferred method for preparing solid-contact ISEs (SC-ISEs) due to its ease and scalability. However, while there are many solution-processable cationic SC-ISEs, this remains a challenge for anionic SC-ISEs due to poorer compatibility with the solid-contacts. Many anionic SC-ISEs are still prepared by complex techniques, such as electropolymerisation of the solid-contact. Thus, strategies for solution-processable solid-contacts which can interface well with anionic ion-selective membrane (ISMs) are required. Here, we report the fabrication of a fully-solution-processable chloride (Cl-) SC-ISE by anion exchange of poly(3,4-ethylenedioxythiophene)-polyethylene glycol (PEDOT-PEG) solid-contact before drop-casting the ISM. Significant improvement in sensitivity was observed after PEDOT-PEG anion exchange, with the optimal SC-ISE exhibiting near-Nernstian response (-53.3 ± 0.5 mV/decade, versus -33.4 ± 1.8 mV/decade for the unexchanged SC-ISE) across a wide dynamic range (0.05 M-6.03 μM). Our SC-ISE also exhibited excellent selectivity against phosphate (H2PO4-), bicarbonate (HCO3-) and acetate (CH3CO2-) and could be utilized with minimal conditioning time and for prolonged usage. Finally, given the importance of Cl- sensing in healthcare, we also demonstrated the potential of our Cl- SC-ISE in sensing multiple synthetic biological samples such as sweat, urine and blood, and real human sweat (forearm samples). This work not only demonstrates the versatility of our anion exchange protocol, but also furthers the understanding of the different enhancement mechanisms - sensitivity or selectivity - depending on whether an ionophore was present in the ISM. We showed that regardless of the mechanism, our simple and efficient protocol could mitigate the issue of the original underperformance and can thus be readily extended to the scalable preparation of multiple types of anion-selective SC-ISEs.

Advancing multiplexed ion monitoring techniques: The development of integrated thermally drawn polymer fiber-based ion probes

The monitoring of ion homeostasis in vivo is of paramount importance due to its critical functions in biological systems. However, current leading technologies for creating ion-selective electrodes often fall short of the requirements for in vivo applications in terms of multiplexity, miniaturization, and flexibility. To address this gap, we introduce an integrated multiplexed ion monitoring probe created from thermally drawn multi-electrode polymer fiber, aimed at enhancing in vivo ion homeostasis studies. This probe employs a carbon nanofiber (CNF)/graphene composite as the sensing material, utilizing a thermal drawing process, laser machining, and material functionalization to fabricate multiplexed ion probes. Our design incorporates electrodes on micron-scale fibers for sensing Na+, K+ and Cl- ions, alongside an electrode for electrophysiology recording, achieving excellent sensitivity, stability, selectivity, and reversibility in distilled water and artificial cerebrospinal fluid solutions (aCSF). These results demonstrate the potential of the probe for future in vivo applications.

Sensitivity-enhanced potentiometric measurement by incorporating graphitic carbon nitride into the ion-to-electron transducer of potassium ion-selective electrodes

In recent years, wearable sweat sensors have garnered significant attention for real-time monitoring of human physiological information because of their ability to continuously and non-invasively detect multiple sweat biomarkers. Among these, potentiometric sensors stand out for their low power consumption, low cost, compact design, and real-time monitoring capabilities, making them an ideal alternative for sweat analysis. However, enhancing the sensitivity of ion-selective electrodes (ISEs), a critical parameter of potentiometric sensors, remains a challenging research focus. In this work, the sensitivity of K+ ISEs was significantly enhanced by doping two-dimensional nanoparticles graphitic carbon nitride (g-C₃N₄) into the ion-to-electron transducer of the electrode via electrodeposition. The calibration curve slope of the K+ potentiometric sensors with doped g-C3N4 reached 59.6 mV/dec, representing a 33% increase in sensitivity compared to the control sensor without g-C₃N₄. Furthermore, the developed sensors demonstrated excellent repeatability, and anti-interference capabilities. Finally, the feasibility of the prepared sensors was further validated in artificial sweat. The large specific surface area of g-C₃N₄ combined with the excellent conductivity of PEDOT: PSS, significantly improved the sensitivity of ISEs in this study. This innovative approach paves a new avenue for the application of two-dimensional materials in potentiometric sensors, potentially advancing the field of real-time sweat analysis.

A novel type of planar reference electrodes based on ionic liquids

Although electrochemical sensors gained a lot of popularity through recent years, there is very little research on sensors with IL-based reference electrodes. This type of reference electrodes might be the ultimate solution for problem of RE miniaturization. In this paper a novel type of printed reference electrodes based on ionic liquids are presented. The potential stability of electrodes with membranes containing two new ILs with promising properties, namely 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMIM+FAP-) and 1-(2-methoxyethyl)-1-methylpyrrolidin-1-ium tris(pentafluoroethyl)trifluorophosphate (PYR(2o1,1)+FAP-), was investigated. Reference membranes were implemented in classic electrodes with internal electrolyte, as well as deposited on planar transducers with electrodes fabricated using screen printing or aerosol jet printing. Membranes were deposited via drop-casting or by using aerosol jet printer, to form fully printed reference electrodes. It was found that while both tested ionic liquids performed similarly, the use of (PYR(2o1,1)+FAP-) resulted in better potential stability. Planar IL-based electrode was finally used as a reference electrode in a simple pH sensor, enabling the detection of pH changes.

Worth your sweat: wearable microfluidic flow rate sensors for meaningful sweat analytics

Wearable microfluidic sweat sensors could play a major role in the future of monitoring health and wellbeing. Sweat contains biomarkers to monitor health and hydration status, and it can provide information on drug intake, making it an interesting non-invasive alternative to blood. However, sweat is not created in excess, and this requires smart sweat collection strategies to handle small volumes. Microfluidic solutions are commonly employed which use capillary action or evaporation to drive flow. In current literature about sweat analytics, the emphasis lies predominantly on developing the sensors for measuring the composition of sweat. Yet, solely measuring sweat composition does not suffice, because the composition varies due to inter- and intra-individual differences in sweat rate. The measurement of sweat rate is thus crucial for enabling a reliable interpretation and standardisation of this data. Recently, more wearable sweat sensors, also integrating a means of measuring flow, have been developed. This manuscript reviews state-of-the-art sweat collection strategies and flow rate measuring techniques. Generally, flow rate measurements are performed by impedimetric or capacitive methods. However, these techniques can be impaired due to limited lifetime and signal interference from changing ionic contents in sweat. Discrete measurement techniques, such as impedance measurements of an advancing fluid front with interdigitated electrodes, calorimetric and colorimetric techniques can be very reliable, because they selectively measure flow. However, one should take the available size, intended application and compatibility with other sensors into account. Overall, accurate flow rate sensors integrated in reliable microfluidic sweat sensor platforms will enable the standardisation of sweat measurements to unlock the potential of sweat analytics in advancing physiological research, personalized diagnostics and treatment of diseases.

Next-Generation Potentiometric Sensors: A Review of Flexible and Wearable Technologies

In recent years, the field of wearable sensors has undergone significant evolution, emerging as a pivotal topic of research due to the capacity of such sensors to gather physiological data during various human activities. Transitioning from basic fitness trackers, these sensors are continuously being improved, with the ultimate objective to make compact, sophisticated, highly integrated, and adaptable multi-functional devices that seamlessly connect to clothing or the body, and continuously monitor bodily signals without impeding the wearer's comfort or well-being. Potentiometric sensors, leveraging a range of different solid contact materials, have emerged as a preferred choice for wearable chemical or biological sensors. Nanomaterials play a pivotal role, offering unique properties, such as high conductivity and surface-to-volume ratios. This article provides a review of recent advancements in wearable potentiometric sensors utilizing various solid contacts, with a particular emphasis on nanomaterials. These sensors are employed for precise ion concentration determinations, notably sodium, potassium, calcium, magnesium, ammonium, and chloride, in human biological fluids. This review highlights two primary applications, that is, (1) the enhancement of athletic performance by continuous monitoring of ion levels in sweat to gauge the athlete's health status, and (2) the facilitation of clinical diagnosis and preventive healthcare by monitoring the health status of patients, in particular to detect early signs of dehydration, fatigue, and muscle spasms.

Ion Sensing via Modulation of Charge Transfer in Donor-pi-Acceptor Molecules: Structure, Mechanism & Photophysical Aspects

This review article highlights the importance of novel charge transfer (CT) sensing approach for the detection of ions which are crucial from environmental and biological point of view. The importance, principles of charge transfer, ion sensing, its different types, and its basic process will all be covered here. The strategy has been reported with enormous sensitivity and fast signaling response owing to the fact that strong electronic connection communication exists between donor (D) and acceptor (A) part. Important discoveries made since 2010 will be examined. Herein, we will showcase the binding constants, conditions employed for sensing, and limit of detection of crucial ions via CT based sensors that researchers have bough forth for real-time applications. Additionally, the focus will be on the mechanistic aspects and signaling response as a result of the interaction between ion and sensor molecule.

Nanoionics enabled atomic point contact construction and quantum conductance effects

The miniaturization of electronic devices is important for the development of high-density and function-integrated information devices. Atomic-point-contact (APC) structures refer to narrow contact areas formed by one or more atoms between two conductive electrodes that produce quantum conductance effects when the electrons pass through the APC channel, providing a new development path for the miniaturization of information devices. Recently, nanoionics has enabled the electric field reconfiguration of APC structures in solid-state electrolytes, offering new approaches to controlling the quantum conductance states, which may lead to the development of emerging information technologies with low power consumption, high speed, and high density. This review provides an overview of APC structures with a focus on the fabrication methods enabled by nanoionics technology. In particular, the advantages of electric field-driven nanoionics in the construction of APC structures are summarized, and the influence of external fields on quantum conductance effects is discussed. Recent studies on electric field regulation of APC structures to achieve precise control of quantum conductance states are also reviewed. The potential applications of quantum conductance effects in memory, computing, and encryption-related information technologies are further explored. Finally, the challenges and future prospects of quantum conductance effects in APC structures are discussed.

Calibration-free and ready-to-use wearable electroanalytical reporting system (r-WEAR) for long-term remote monitoring of electrolytes markers

A major bottleneck in the development of wearable ion-selective sensors is the inherent conditioning and calibration procedures at the user's end due to the signal's instability and non-uniformity. To address this challenge, we developed a strategy that integrates three interdependent materials and device engineering approaches to realize a Ready-to-use Wearable ElectroAnalytical Reporting system (r-WEAR) for reliable electrolytes monitoring. The strategy collectively utilized (1) finely-configured diffusion-limiting polymers to stabilize the electromotive force in the electrodes, (2) a uniform electrical induction in electrochemical cells to normalize the open-circuit potential (OCP), and (3) an electrical shunt to maintain the OCP across the entire sensor in the r-WEAR. The approaches jointly enable fabrication of homogeneously stable and uniform ion-selective sensors, eliminating common conditioning and calibration practices. As a result, the r-WEAR demonstrated a signal's variation down to ±1.99 mV with a signal drift of 0.5 % per hour (0.12 mV h-1) during a 12-h continuous measurement of 10 sensors and a signal drift as low as 13.3 μV h-1 during storage. On-body evaluations of the r-WEAR for four days without conditioning and re-/calibration further validated the sensor's performance in realistic settings, indicating its remarkable potential for practical usage in a user operation-free manner in wearable healthcare applications.

Recent Advances in Wearable Sweat Sensor Development

Wearable sweat sensors for detecting biochemical markers have emerged as a transformative research area, with the potential to revolutionize disease diagnosis and human health monitoring. Since 2016, a substantial body of pioneering and translational work on sweat biochemical sensors has been reported. This review aims to provide a comprehensive summary of the current state-of-the-art in the field, offering insights and perspectives on future developments. The focus is on wearable microfluidic platforms for sweat collection and delivery and the analytical chemistry applicable to wearable devices. Various microfluidic technologies, including those based on synthetic polymers, paper, textiles, and hydrogels, are discussed alongside diverse detection methods such as electrochemistry and colorimetry. Both the advantages and current limitations of these technologies are critically examined. The review concludes with our perspectives on the future of wearable sweat sensors, with the goal of inspiring new ideas, innovations, and technical advancements to further the development and practical application of these devices in promoting human health.

Wearable Electrochemical Biosensors for Advanced Healthcare Monitoring

Recent advancements in wearable electrochemical biosensors have opened new avenues for on-body and continuous detection of biomarkers, enabling personalized, real-time, and preventive healthcare. While glucose monitoring has set a precedent for wearable biosensors, the field is rapidly expanding to include a wider range of analytes crucial for disease diagnosis, treatment, and management. In this review, recent key innovations are examined in the design and manufacturing underpinning these biosensing platforms including biorecognition elements, signal transduction methods, electrode and substrate materials, and fabrication techniques. The applications of these biosensors are then highlighted in detecting a variety of biochemical markers, such as small molecules, hormones, drugs, and macromolecules, in biofluids including interstitial fluid, sweat, wound exudate, saliva, and tears. Additionally, the review also covers recent advances in wearable electrochemical biosensing platforms, such as multi-sensory integration, closed-loop control, and power supply. Furthermore, the challenges associated with critical issues are discussed, such as biocompatibility, biofouling, and sensor degradation, and the opportunities in materials science, nanotechnology, and artificial intelligence to overcome these limitations.

Laser-Induced Graphene Electrodes for Flexible pH Sensors

In the growing field of personalized medicine, non-invasive wearable devices and sensors are valuable diagnostic tools for the real-time monitoring of physiological and biokinetic signals. Among all the possible multiple (bio)-entities, pH is important in defining health-related biological information, since its variations or alterations can be considered the cause or the effect of disease and disfunction within a biological system. In this work, an innovative (bio)-electrochemical flexible pH sensor was proposed by realizing three electrodes (working, reference, and counter) directly on a polyimide (Kapton) sheet through the implementation of CO2 laser writing, which locally converts the polymeric sheet into a laser-induced graphene material (LIG electrodes), preserving inherent mechanical flexibility of Kapton. A uniform distribution of nanostructured PEDOT:PSS was deposited via ultrasonic spray coating onto an LIG working electrode as the active material for pH sensing. With a pH-sensitive PEDOT coating, this flexible sensor showed good sensitivity defined through a linear Nernstian slope of (75.6 ± 9.1) mV/pH, across a pH range from 1 to 7. We demonstrated the capability to use this flexible pH sensor during dynamic experiments, and thus concluded that this device was suitable to guarantee an immediate response and good repeatability by measuring the same OCP values in correspondence with the same pH applied.

Potentiometric determination of the local anesthetic procaine in pharmaceutical samples

In this study, a new potentiometric sensor was developed for the determination of the local anesthetic drug procaine in pharmaceutical samples. Procaine (Pr)-Tetraphenlyborate (TPB) ion-pair was synthesized and used as a sensor material. Potentiometric sensors using the synthesized ion pair (Pr-TPB), poly(vinyl chloride) (PVC) and o-nitrophenyloctyl ether (o-NPOE) in different proportions were prepared and their performance properties were tested. Among the prepared sensors, the best potentiometric response characteristics were obtained with the sensor composition Pr-TPB:PVC:o-NPOE in the ratio of 6.0:32.0:62.0 (w/w %). The new procaine sensor developed in the present study had a near-Nernstian behavior of 54.1 ± 3.3 mV/per decade and a low detection limit of 3.18 × 10-5 mol L-1 in the concentration range of 1.0 × 10-1-1.0 × 10-4 mol L-1. Additionally, the sensor had a response time of less than 10 s and could work in a wide pH range for two different concentration values without being affected by pH changes. Finally, the new procaine potentiometric sensor was used to detect procaine in injection samples and successfully determined procaine concentrations with high recoveries.

Surface Blocking-Based Potentiometric Biosensor for Detection of E. coli ATCC 15597 Using Phage MS2 as a Receptor

Nowadays, using a potentiometric ion sensor to achieve detection of biological analytes is still a big challenge, since these analytes usually do not yield a measurable ion signal. To address this challenge, a simple and robust potentiometric sensing protocol based on a delayed Nernstian response is proposed for the label-free detection of biological analytes. The proposed sensor platform is composed of two layers: the surface recognition layer and the indicator-ion-selective electrode (ISE) membrane layer. It is based on a surface blocking mechanism in which the surface recognition interactions between the surface recognition element and the target can impede the diffusion of the indicator ion from the aqueous phase to the sensing membrane phase to reach the final Nernstian-response equilibrium, thus resulting in a delayed potential change. Such a potential change could be used to measure the concentration of a biological target in the sample. Thus, a sensing system was designed by using phage MS2, its host bacterium Escherichia coli ATCC 15597(abbreviated as E. coli H), and a solid-contact butyrylcholine ISE as a surface recognition element, a target, and an indicator electrode, respectively. This new concept offers a simple, sensitive, and extremely selective potentiometric method for detection of E. coli H with a detection limit of 100 CFU mL-1. It can be expected that the present approach may pave the way to using ISEs to detect various important nonionic biological targets in clinical and environmental applications.

Demonstration of a Validated Direct Current Wearable Device for Monitoring Sweat Rate in Sports

Sweat rate magnitude is a desired outcome for any wearable sensing patch dedicated to sweat analysis. Indeed, sweat rate values can be used two-fold: self-diagnosis of dehydration and correction/normalization of other physiological metrics, such as Borg scale, VO2, and different chemical species concentrations. Herein, a reliable sweat rate belt device for sweat rate monitoring was developed. The device measures sweat rates in the range from 1.0 to 5.0 µL min-1 (2 to 10 µL min-1 cm-2), which covers typical values for humans. The working mechanism is based on a new direct current (DC) step protocol activating a series of differential resistance measurements (spatially separated by 800 µm) that is gradually initiated by the action of sweat, which flows along a customized microfluidic track (~600 µm in width, 10 mm in length, and 235 µm in thickness). The device has a volumetric capacity of ~16 µL and an acquisition frequency between 0.010 and 0.043 Hz within the measured sweat rate range. Importantly, instead of using a typical and rather complex AC signal interrogation and acquisition, we put forward the DC approach, offering several benefits, such as simplified circuit design for easier fabrication and lower costs, as well as reduced power consumption and suitability for wearable applications. For the validation, either the commercial sweat collector (colorimetric) or the developed device was performed. In five on-body tests, an acceptable variation of ca. 10% was obtained. Overall, this study demonstrates the potential of the DC-based device for the monitoring of sweat rate and also its potential for implementation in any wearable sweat platform.

Flexible ammonium ion-selective electrode based on inkjet-printed graphene solid contact

Miniaturization and mass-production of potentiometric sensor systems is paving the way towards distributed environmental sensing, on-body measurements and industrial process monitoring. Inkjet printing is gaining popularity as a highly adaptable and scalable production technique. Presented here is a scalable and low-cost route for flexible solid-contact ammonium ion-selective electrode fabrication by inkjet printing. Utilization of inkjet-printed melamine-intercalated graphene nanosheets as the solid-contact material significantly improved charge transport, while evading the detrimental water-layer formation. External polarization was investigated as a means of improving the inter-electrode reproducibility: the standard deviations of E0 values were reduced after electrode polarization, the linear region of the response was extended to the range 10-1-10-6 M of NH4Cl and LODs reduced to 0.88 ± 0.17 μM. Finally, we have shown that the electrodes are adequate for measurements in a complex real sample: ammonium concentration was determined in landfill leachate water, with less than 4 % deviation from the reference method.

Microfluidic Wearable Electrochemical Sensor Based on MOF-Derived Hexagonal Rod-Shaped Porous Carbon for Sweat Metabolite and Electrolyte Analysis

Wearable sensors enable the noninvasive continuous analysis of biofluid, which is of great importance for healthcare monitoring. In this work, a wearable sensor was seamlessly integrated with a microfluidic chip which was prepared by a three-dimensional printing technology for noninvasive and multiplexed analysis of metabolite and electrolytes in human sweat. The microfluidic chip could enable rapid sampling of sweat, which avoids the sweat evaporation and contamination. Using a Zn metal-organic framework as a sacrificial template, the hexagonal rod-shaped porous carbon nanorod (PCN) with high porosity, a large specific surface area, and excellent conductivity was synthesized and exhibited the robust electrocatalytic ability of uric acid (UA) oxidation. Therefore, the PCN-based sensor showed high sensitivity and good selectivity of UA with a wide linear range of 10-200 μM and a low detection limit of 4.13 μM. Meanwhile, the potentiometry-based ion-selective electrode was constructed for detection of pH and K+, respectively, with good sensitivity, selectivity, reproducibility, and stability. In addition, the testing under different bending states demonstrated that mechanical deformation had little effect on the electrochemical performance of the wearable sensors. Furthermore, we evaluated the utility of the wearable sensor for multiplexed real-time analysis of UA, pH, and K+ in sweat during aerobic exercise, and the effect of the amount of consumed purine-rich foods on uric acid metabolite levels in sweat and urine was further investigated. The relationship between urine UA and sweat UA was obtained. Overall, this wearable sensor enables multiple electrolyte and metabolite analysis in different noninvasive biofluids, suggesting its potential application in personalized disease prevention.

3D-Printed Transducers for Solid Contact Potentiometric Ion Sensors: Improving Reproducibility by Fabrication Automation

3D printing technology has become attractive in the development of electrochemical sensors as it offers automation in fabrication, customization on-demand, and reproducibility, among other features. Nonetheless, to date, solid contact potentiometric ion sensors have remained overlooked using this technology. Thus, the novelty of this work relies on demonstrating for the first time the usefulness of the multimaterial 3D printing approach to manufacture potentiometric ion-selective electrodes. The significance is indeed twofold. First, we discovered that by using the polyethylene terephthalate glycol (PETg) and polylactic acid-carbon black (PLA-CB) filaments together with a rational electrode design containing a well to accommodate the ion-selective membrane, a tight seal among all of the sensing materials is obtained. Importantly, this has mainly impacted the electrode-to-electrode reproducibility (ERSD0 ± 3 mV). Second, 75 ready-to-use electrodes can be printed in less than 3.5 h in a completely automated manner at a cost of ∼0.32 €/sensor. This feature may positively impact the suitability of further scaled-up production as well as the possibility of application in low-resource contexts. Overall, the presented outcomes are expected to encourage certain research directions to adopt using multimaterial 3D-printing approaches for producing highly reproducible solid contact potentiometric ion-selective electrodes, but are not restricted to them.

All-solid-state K+ sensing array based on Au@polystyrene nanocomposites

All-solid-state ion selective electrodes (ASS-ISEs) are easy to miniaturize and array, meeting the needs of home sensing devices. However, ASS-ISEs still faces challenges in accuracy and stability due to basic potential changes caused by non-specific adsorption of charged background compositions and the complex electrode preparation steps. To this end, our group successfully subtracted the background signal by integrating a self-calibrating channel in the sensing array and simplified the electrode preparation steps by preparing multi-functional PS-Au nanocomposites. However, the uniformity and gold content of PS-Au nanocomposites are difficult to control, so Au@PS nanocomposites are prepared as sensor materials in this paper to further reduce the differences between batches of electrodes. K+ Au@PS sensing array can be obtained by directly dropping Au@PS nanocomposites on the screen-printed carbon electrodes (SPCEs), which shows a near Nernstian behavior in the range 1.0 × 10-3 M to 0.3 M and good reproducibility in real sample testing. The detection results by K+ Au@PS sensing array for K+ in human morning urine agreed well with that tested by ICP-AES, which make the K+-ASS-ISE suitable for home health monitoring.

Highly Sensitive Potentiometric pH Sensor Based on Polyaniline Modified Carbon Fiber Cloth for Food and Pharmaceutical Applications

This study introduces a potentiometric pH sensor that is extremely sensitive and specifically designed for food and pharmaceutical applications. The sensor utilizes a pH-sensitive interface fabricated by electropolymerizing polyaniline (PANI) on carbon fiber cloth (CFC). Structural and morphological analyses of PANI-CFC and CFC have been conducted by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). The investigation of the functional groups was conducted by using Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The electrochemical characteristics were assessed by utilization of cyclic voltammetry (CV) and open-circuit potential (OCP) measurements in a three-electrode configuration. The sensor exhibited a sensitivity of 60.9 mV/pH, while retaining consistent performance within the pH range of 4 to 12. The repeatability and robustness of the sensors were verified. The accuracy of the PANI-CFC sensor was confirmed by validation using real samples, demonstrating its compatibility with commercially available pH sensors. The application of density functional theory (DFT) calculations revealed an interaction energy of -173.2886 kcal/mol, indicating a strong affinity of H+ ions towards PANI-CFC electrode. Further investigation was conducted to examine the chemical reactivity of PANI, revealing a HOMO-LUMO energy gap of -0.98 eV. This study highlights the PANI-CFC sensor as a reliable and efficient pH-sensing platform for food and pharmaceuticals applications, performing robustly in both laboratory and real-world settings.

Insights into solid-contact ion-selective electrodes based on laser-induced graphene: Key performance parameters for long-term and continuous measurements

This work aims to serve as a comprehensive guide to properly characterize solid-contact ion-selective electrodes (SC-ISEs) for long-term use as they advance toward calibration-free sensors. The lack of well-defined SC-ISE performance criteria limits the ability to compare results and track progress in the field. Laser-induced graphene (LIG) is a rapid and scalable method that, by adjusting the CO2 laser parameters, can create LIG substrates with tunable surface properties, including wettability, surface chemistry, and morphology. Herein, we fabricate laser-induced graphene (LIG) solid-contact electrodes using different laser settings and subsequently convert them into ion-selective sensors using a potassium-selective membrane. We measure the aforementioned tunable surface properties and correlate them with resultant low-frequency capacitance and water layer formation in an effort to pinpoint their effects on the sensitivity (Nernstian response), reproducibility (E°' variation), and potential stability of the LIG-based SC-ISEs. More specifically, we demonstrate that the surface wettability of the LIG substrate, which can be tuned by controlling the lasing parameters, can be modified to exhibit hydrophobic (contact angle > 90°) and even highly hydrophobic surfaces (contact angle ≈ 130°) to help reduce sensor drift. Recommendations are also provided to ensure proper and robust characterization of SC-ISEs for long-term and continuous measurements. Ultimately, we believe that a comprehensive understanding of the correlation between LIG tunable surface properties and SC-ISE performance can be used to improve the electrochemical behavior and stability of SC-ISEs designed with a wide range of materials beyond LIG.

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

Potassium Ion-Selective Electrodes with BME-44 Ionophores Covalently Attached to Condensation-Cured Silicone Membranes

For ion-selective electrodes (ISEs) to be employed in wearable and implantable applications, the ion-selective membrane components should be biocompatible, and leaching of components, such as plasticizer or ionophore, out of the sensing membrane should be inhibited. To achieve this, we employed a plasticizer-free silicone as the membrane matrix and synthesized as the ionophore a derivative of the bis-crown ether based potassium ionophore BME-44, incorporating a triethoxysilyl functional group that covalently attaches to condensation-cured silicones during the curing process. Soxhlet extraction of these membranes with dichloromethane shows that up to 96% of the ionophore is attached to the silicone membrane during curing. We found that the covalently attachable BME-44 derivative can inadvertently adsorb onto high surface area carbon solid contacts before attaching to the silicone matrix if the curing of the silicone is performed in the presence of the high surface area carbon, resulting in depletion of ionophore from the membrane and yielding solid-contact ISEs with poor selectivity. In contrast, we observed Nernstian responses to K+ in plasticizer-free silicone-based K+ ISMs with either mobile BME-44 or the covalently attachable BME-44 derivative when the membranes were prepared on octane-thiol coated gold electrodes, where ionophore adsorption does not occur to a noticeable extent. As compared with ISMs doped with the mobile BME-44, ISMs prepared with the covalently attachable BME-44 derivative have better selectivity for K+ vs Na+ (log ⁡ K K + , N a + values of -3.54 and <- 4.05 for mobile and covalently attachable BME-44, respectively) and lower resistance. This can be explained by a more homogeneous incorporation of the covalently attachable BME-44 derivative into the silicone matrix than is the case for the mobile BME-44.

Smart theranostics for wound monitoring and therapy

To overcome the challenges of poor wound diagnosis and limited clinical efficacy of current wound management, wound dressing materials with the aim of monitoring various biomarkers vital to the wound healing process such as temperature, pH, glucose concentration, and reactive oxygen species (ROS) and improving the therapeutic outcomes have been developed. These innovative theranostic dressings are smartly engineered using stimuli-responsive biomaterials to monitor and regulate local microenvironments and deliver cargos to the wound sites in a timely and effective manner. This review provides an overview of recent advances in novel theranostics for wound monitoring and therapy as well as giving insights into the future treatment of wounds via smart design of theranostic materials.

A laser-Engraved Wearable Electrochemical Sensing Patch for Heat Stress Precise Individual Management of Horse

In point-of-care diagnostics, the continuous monitoring of sweat constituents provides a window into individual's physiological state. For species like horses, with abundant sweat glands, sweat composition can serve as an early health indicator. Considering the salience of such metrics in the domain of high-value animal breeding, a sophisticated wearable sensor patch tailored is introduced for the dynamic assessment of equine sweat, offering insights into pH, potassium ion (K+), and temperature profiles during episodes of heat stress and under normal physiological conditions. The device integrates a laser-engraved graphene (LEG) sensing electrode array, a non-invasive iontophoretic module for stimulated sweat secretion, an adaptable signal processing unit, and an embedded wireless communication framework. Profiting from an admirable Truth Table capable of logical evaluation, the integrated system enabled the early and timely assessment for heat stress, with high accuracy, stability, and reproducibility. The sensor patch has been calibrated to align with the unique dermal and physiological contours of equine anatomy, thereby augmenting its applicability in practical settings. This real-time analysis tool for equine perspiration stands to revolutionize personalized health management approaches for high-value animals, marking a significant stride in the integration of smart technologies within the agricultural sector.

Integrating microneedles and sensing strategies for diagnostic and monitoring applications: The state of the art

Microneedles (MNs) offer minimally-invasive access to interstitial fluid (ISF) - a potent alternative to blood in terms of monitoring physiological analytes. This property is particularly advantageous for the painless detection and monitoring of drugs and biomolecules. However, the complexity of the skin environment, coupled with the inherent nature of the analytes being detected and the inherent physical properties of MNs, pose challenges when conducting physiological monitoring using this fluid. In this review, we discuss different sensing mechanisms and highlight advancements in monitoring different targets, with a particular focus on drug monitoring. We further list the current challenges facing the field and conclude by discussing aspects of MN design which serve to enhance their performance when monitoring different classes of analytes.

Improvement of the Upper Detection Limit of Ionophore-Based H+-Selective Electrodes: Explanation and Elimination of Apparently Super-Nernstian Responses

The response range of an ion-selective electrode (ISE) has been described by counterion interference at the lower and Donnan failure at the upper detection limit. This approach fails when the potentiometric response at the upper detection limit exhibits an apparently super-Nernstian response, as has been reported repeatedly for H+-selective electrodes. While also observed when samples contain other anions, super-Nernstian responses at low pH are a problem in particular for samples that contain phthalate, a common component of commercial pH calibration solutions. This work shows that coextraction of H+ and a sample anion into the sensing membrane alone does not explain these super-Nernstian responses, even when membrane-internal diffusion potentials are taken into account. Instead, these super-Nernstian responses are explained by the formation of complexes between that anion and at least two protonated ionophore molecules. As demonstrated by experiments and explained with quantitative phase boundary models, the apparently super-Nernstian responses at low pH can be eliminated by restricting the molecular ratio of ionophore and ionic sites. Notably, this conclusion results in recommendations for the optimization of sensing membranes that, in some instances, will conflict with previously reported recommendations from the ionic site theory for the optimization of the lower detection limit. This mechanistic insight is key to maximizing the response range of these ionophore-based ISEs.

Chronopotentiometric Nanopore Sensor Based on a Stimulus-Responsive Molecularly Imprinted Polymer for Label-Free Dual-Biomarker Detection

The development of sensors for detection of biomarkers exhibits an exciting potential in diagnosis of diseases. Herein, we propose a novel electrochemical sensing strategy for label-free dual-biomarker detection, which is based on the combination of stimulus-responsive molecularly imprinted polymer (MIP)-modified nanopores and a polymeric membrane chronopotentiometric sensor. The ion fluxes galvanostatically imposed on the sensing membrane surface can be blocked by the recognition reaction between the target biomarker in the sample solution and the stimulus-responsive MIP receptor in the nanopores, thus causing a potential change. By using two external stimuli (i.e., pH and temperature), the recognition abilities of the stimulus-responsive MIP receptor can be effectively modulated so that dual-biomarker label-free chronopotentiometric detection can be achieved. Using alpha fetoprotein (AFP) and prostate-specific antigen (PSA) as model biomarkers, the proposed sensor offers detection limits of 0.17 and 0.42 ng/mL for AFP and PSA, respectively.

Graphene Oxide-Poly(vinyl alcohol) Hydrogel-Coated Solid-Contact Ion-Selective Electrodes for Wearable Sweat Potassium Ion Sensing

Solid-contact ion-selective electrodes (SC-ISEs) with ionophore-based polymer-sensitive membranes have been the major devices in wearable sweat sensors toward electrolyte analysis. However, the toxicity of ionophores in ion-selective membranes (ISMs), for example, valinomycin (K+ ion carrier), is a significant challenge, since the ISM directly contacts the skin during the tests. Herein, we report coating a hydrogel of graphene oxide-poly(vinyl alcohol) (GO-PVA) on the ISM to fabricate hydrogel-based SC-ISEs. The hydrogen bond interaction between GO sheets and PVA chains could enhance the mechanical strength through the formation of a cross-linking network. Comprehensive electrochemical tests have demonstrated that hydrogel-coated K+-SC-ISE maintains Nernstian response sensitivity, high selectivity, and anti-interference ability compared with uncoated K+-SC-ISE. A flexible hydrogel-based K+ sensing device was further fabricated with the integration of a solid-contact reference electrode, which has realized the monitoring of sweat K+ in real time. This work highlights the possibility of hydrogel coating for fabricating biocompatible wearable potentiometric sweat electrolyte sensors.

Wireless wearable potentiometric sensor for simultaneous determination of pH, sodium and potassium in human sweat

This paper reports on the development of a flexible-wearable potentiometric sensor for real-time monitoring of sodium ion (Na+), potassium ion (K+), and pH in human sweat. Na0.44MnO2, polyaniline, and K2Co[Fe(CN)6] were used as sensing materials for Na+, H+ and K+ monitoring, respectively. The simultaneous potentiometric Na+, K+, and pH sensing were carried out by the developed sensor, which enables signal collection and transmission in real-time to the smartphone via a Wi-Fi access point. Then, the potentiometric responses were evaluated by a designed android application. Na+, K+, and pH sensors illustrated high sensitivity (59.7 ± 0.8 mV/decade for Na+, 57.8 ± 0.9 mV/decade for K+, and 54.7 ± 0.6 mV/pH for pH), excellent stability, and good batch-to-batch reproducibility. The results of on-body experiments demonstrated that the proposed platform is capable of real-time monitoring of the investigated ions.

A Multichannel Screen-Printed Carbon Electrode Based on Fluorinated Poly(3-octylthiophene-2,5-diyl) and Purified Mesoporous Carbon Black Simultaneously Detects Na+, K+, Ca2+, and NO2. ACS OMEGA 2024;9:18238-18248. [PMID: 38680364 PMCID: PMC11044230 DOI: 10.1021/acsomega.3c10471] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Preparation of nanocomposites based on fluorinated poly(3-octylthiophene-2,5-diyl) (POTF) and purified mesoporous carbon black (PMCB) as the solid-contact layer of a screen-printed carbon electrode (SPCE) is proposed. POTF is used as a dispersant for PMCB. The obtained nanocomposites possess unique characteristics including high conductivity, capacitance, and stability. The SPCE based on POTF and PMCB is characterized by electrochemical impedance spectroscopy and chronopotentiometry, demonstrating simultaneous detection of Na+, K+, Ca2+, and NO2- ions with detection limits of 10-6.5, 10-6.4, 10-6.7, and 10-6.3 M, respectively. Water layer and anti-interference tests revealed that the electrode has high hydrophobicity, and the static contact angle is >140°. The electrode shows excellent selectivity, repeatability, reproducibility, and stability and is not easily affected by light, O2, or CO2.

Improvement of the selectivity of a molecularly imprinted polymer-based potentiometric sensor by using a specific functional monomer

Potentiometric sensors based on the molecularly imprinted polymers (MIPs) as the receptors have been successfully developed for determination of various organic and biological species. However, these MIP receptors may suffer from problems of low selectivity. Especially, it would be difficult to distinguish the target analyte from its structurally similar interferents. In this work, we propose a novel strategy that using specific functional monomer to fabricate MIP with high selectivity towards the target molecule. The density functional theory calculations are used to investigate the interactions between the template and the functional monomer. The binding energy between the template and functional monomer can be used as the criterion for identifying the optimal monomer. As a proof-of-concept experiment, bisphenol A (BPA) is chosen as the template and the MIP is synthesized by the precipitation polymerization method using the specific allyl-β-cyclodextrin (allyl-β-CD) with high affinity towards BPA as the functional monomer. The high-affinity MIP is employed as the receptor for the construction of the potentiometric sensor. The proposed potentiometric sensor based on the MIP using allyl-β-CD as the functional monomer shows an improved response performance in terms of selectivity and sensitivity compared to the conventional potentiometric sensor based on the MIP with the common monomer (i.e., methacrylic acid). This allyl-β-CD MIP-based potentiometric sensor shows a detection limit of 0.29 μM for BPA, which is about one order of magnitude lower than that obtained by the conventional MIP-based potentiometric sensor. We believe that utilizing a functional monomer with specific recognition ability towards target in the fabrication of MIP could provide an appealing way to construct highly selective MIP-based electrochemical and optical sensors.

Multifunctional Siloxene-Decorated Laser-Inscribed Graphene Patch for Sweat Ion Analysis and Electrocardiogram Monitoring

Potentiometric detection in complex biological fluids enables continuous electrolyte monitoring for personal healthcare; however, the commercialization of ion-selective electrode-based devices has been limited by the rapid loss of potential stability caused by electrode surface inactivation and biofouling. Here, we describe a simple multifunctional hybrid patch incorporating an Au nanoparticle/siloxene-based solid contact (SC) supported by a substrate made of laser-inscribed graphene on poly(dimethylsiloxane) for the noninvasive detection of sweat Na+ and K+. These SC nanocomposites prevent the formation of a water layer during ion-to-electron transfer, preserving 3 and 5 μV/h potential drift for the Na+ and K+ ion-selective electrodes, respectively, after 13 h of exposure. The lamellar structure of the siloxene sheets increases the SC area. In addition, the electroplated Au nanoparticles, which have a large surface area and excellent conductivity, further increased the electric double-layer capacitance at the interface between the ion-selective membranes and solid-state contacts, thus facilitating ion-to-electron transduction and ultimately improving the detection stability of Na+ and K+. Furthermore, the integrated temperature and electrocardiogram sensors in the flexible patch assist in monitoring body temperature and electrocardiogram signals, respectively. Featuring both electrochemical ion-selective and physical sensors, this patch offers immense potential for the self-monitoring of health.

Electrochemical sensing fibers for wearable health monitoring devices

Real-time monitoring of health conditions is an emerging strong issue in health care, internet information, and other strongly evolving areas. Wearable electronics are versatile platforms for non-invasive sensing. Among a variety of wearable device principles, fiber electronics represent cutting-edge development of flexible electronics. Enabled by electrochemical sensing, fiber electronics have found a wide range of applications, providing new opportunities for real-time monitoring of health conditions by daily wearing, and electrochemical fiber sensors as explored in the present report are a promising emerging field. In consideration of the key challenges and corresponding solutions for electrochemical sensing fibers, we offer here a timely and comprehensive review. We discuss the principles and advantages of electrochemical sensing fibers and fabrics. Our review also highlights the importance of electrochemical sensing fibers in the fabrication of "smart" fabric designs, focusing on strategies to address key issues in fiber-based electrochemical sensors, and we provide an overview of smart clothing systems and their cutting-edge applications in therapeutic care. Our report offers a comprehensive overview of current developments in electrochemical sensing fibers to researchers in the fields of wearables, flexible electronics, and electrochemical sensing, stimulating forthcoming development of next-generation "smart" fabrics-based electrochemical sensing.

Carbon fiber-based multichannel solid-contact potentiometric ion sensors for real-time sweat electrolyte monitoring

Solid-contact ion-selective electrodes (SC-ISEs) feature miniaturization and integration that have gained extensive attention in non-invasive wearable sweat electrolyte sensors. The state-of-the-art wearable SC-ISEs mainly use polyethylene terephthalate, gold and carbon nanotube fibers as flexible substrates but suffer from uncomfortableness, high cost and biotoxicity. Herein, we report carbon fiber-based SC-ISEs to construct a four-channel wearable potentiometric sensor for sweat electrolytes monitoring (Na+/K+/pH/Cl-). The carbon fibers were extracted from commercial cloth, of which the starting point is addressing the cost and reproducibility issues for flexible SC-ISEs. The bare carbon fiber electrodes exhibited reversible voltammetric and stable impedance performances. Further fabricated SC-ISEs based on corresponding ion-selective membranes disclosed Nernstian sensitivity and anti-interface ability toward both ions and organic species in sweat. Significantly, these carbon fiber-based SC-ISEs revealed high reproducibility of standard potentials between normal and bending states. Finally, a textile-based sensor was integrated with a solid-contact reference electrode, which realized on-body sweat electrolytes analysis. The results displayed high accuracy compared with ex-situ tests by ion chromatography. This work highlights carbon fiber-based multichannel wearable potentiometric ion sensors with low cost, biocompatibility and reproducibility.

Strain-insensitive and multiplexed potentiometric ion sensors via printed PMMA molecular layer

Wearable biomimetic electronics have aroused tremendous attention due to their capability to continuously detect and deliver real-time dynamic physiological signals pertaining to the wearer's environment. However, upon close contact with the human skins, a wearable sensor undergoes mechanical strain which inevitably degrades the electrical performance. To address this issue, we demonstrate a universal design approach for stretchable and multiplexed biosensors that can yield unaltered ion sensing performance under variable mechanical tensile strains, which is achieved by introducing a PMMA molecular layer between stretchable substrate and ion sensors. Such design demonstrates reliable multiplexed ion sensing capability and provides high sensitivity (>50 mV/decade), reliable selectivity, as well as wide working range (0.1-100 mM) for sodium, ammonium, potassium and calcium ions in complex sweat biomarkers. Via this introduced PMMA molecular layer, our sensor even exhibits 95 % electrical performance maintained up to 30 % tensile strain, whereas the mechanical tensile property is far superior to original sensor performance. Besides, the sensors were also utilized for real-time monitoring of ions in sweat to validate its biomedical electronics applications. This sensing platform can be easily extended to other biomimetic sensors to enable stable signal acquisition for biomedical electronics.

Self-powered freestanding multifunctional microneedle-based extended gate device for personalized health monitoring

Online monitoring of prognostic biomarkers is critically important when diagnosing disorders and assessing individuals' health, especially for chronic and infectious diseases. Despite this, current diagnosis techniques are time-consuming, labor-intensive, and performed offline. In this context, developing wearable devices for continuous measurements of multiple biomarkers from body fluids has considerable advantages including availability, rapidity, convenience, and minimal invasiveness over the conventional painful and time-consuming tools. However, there is still a significant challenge in powering these devices over an extended period, especially for applications that require continuous and long-term health monitoring. Herein, a new freestanding, wearable, multifunctional microneedle-based extended gate field effect transistor biosensor is fabricated for online detection of multiple biomarkers from the interstitial fluid including sodium, calcium, potassium, and pH along with excellent electrical response, reversibility, and precision. In addition, a hybrid powering system of triboelectric nanogenerator and solar cell was developed for creating a freestanding, closed-loop platform for continuous charging of the device's battery and integrated with an Internet of Things technology to broadcast the measurements online, suggesting a stand-alone, stable multifunctional tool which paves the way for advanced practical personalized health monitoring and diagnosis.

Wearing the Lab: Advances and Challenges in Skin-Interfaced Systems for Continuous Biochemical Sensing

Continuous, on-demand, and, most importantly, contextual data regarding individual biomarker concentrations exemplify the holy grail for personalized health and performance monitoring. This is well-illustrated for continuous glucose monitoring, which has drastically improved outcomes and quality of life for diabetic patients over the past 2 decades. Recent advances in wearable biosensing technologies (biorecognition elements, transduction mechanisms, materials, and integration schemes) have begun to make monitoring of other clinically relevant analytes a reality via minimally invasive skin-interfaced devices. However, several challenges concerning sensitivity, specificity, calibration, sensor longevity, and overall device lifetime must be addressed before these systems can be made commercially viable. In this chapter, a logical framework for developing a wearable skin-interfaced device for a desired application is proposed with careful consideration of the feasibility of monitoring certain analytes in sweat and interstitial fluid and the current development of the tools available to do so. Specifically, we focus on recent advancements in the engineering of biorecognition elements, the development of more robust signal transduction mechanisms, and novel integration schemes that allow for continuous quantitative analysis. Furthermore, we highlight the most compelling and promising prospects in the field of wearable biosensing and the challenges that remain in translating these technologies into useful products for disease management and for optimizing human performance.

Modern Potentiometric Biosensing Based on Non-Equilibrium Measurement Techniques

Modern potentiometric sensors based on polymeric membrane ion-selective electrodes (ISEs) have achieved new breakthroughs in sensitivity, selectivity, and stability and have extended applications in environmental surveillance, medical diagnostics, and industrial analysis. Moreover, nonclassical potentiometry shows promise for many applications and opens up new opportunities for potentiometric biosensing. Here, we aim to provide a concept to summarize advances over the past decade in the development of potentiometric biosensors with polymeric membrane ISEs. This Concept article articulates sensing mechanisms based on non-equilibrium measurement techniques. In particular, we emphasize new trends in potentiometric biosensing based on attractive dynamic approaches. Representative examples are selected to illustrate key applications under zero-current conditions and stimulus-controlled modes. More importantly, fruitful information obtained from non-equilibrium measurements with dynamic responses can be useful for artificial intelligence (AI). The combination of ISEs with advanced AI techniques for effective data processing is also discussed. We hope that this Concept will illustrate the great possibilities offered by non-equilibrium measurement techniques and AI in potentiometric biosensing and encourage further innovations in this exciting field.

A monolithically integrated in-textile wristband for wireless epidermal biosensing

Textile bioelectronics that allow comfortable epidermal contact hold great promise in noninvasive biosensing. However, their applications are limited mainly because of the large intrinsic electrical resistance and low compatibility for electronics integration. We report an integrated wristband that consists of multifunctional modules in a single piece of textile to realize wireless epidermal biosensing. The in-textile metallic patterning and reliable interconnect encapsulation contribute to the excellent electrical conductivity, mechanical robustness, and waterproofness that are competitive with conventional flexible devices. Moreover, the well-maintained porous textile architectures deliver air permeability of 79 mm s-1 and moisture permeability of 270 g m-2 day-1, which are more than one order of magnitude higher than medical tapes, thus ensuring superior wearing comfort. The integrated in-textile wristband performed continuous sweat potassium monitoring in the range of 0.3 to 40 mM with long-term stability, demonstrating its great potential for wearable fitness monitoring and point-of-care testing.

Recent advances in the peptide-based biosensor designs

Biosensors have rapidly emerged as a high-sensitivity and convenient detection method. Among various types of biosensors, optical and electrochemical are the most commonly used. Conventionally, antibodies have been employed to ensure specific interaction between the transmission material and analytes. However, there has been increasing recognition of peptides as a promising recognition element for biosensor development in recent years. The use of peptides as recognition elements provides high level of specificity, sensitivity, and stability for the detection process. The combination of peptide designs and optical or electrochemical detection methods has significantly improved biosensor efficacy. These advancements present opportunities for developing biosensors with diverse functions that can be used to lay a strong scientific foundation for the development of personalized medicine and various other fields. This paper reviews the recent advancements in the development and application of peptide-based optical and electrochemical biosensors, as well as their prospects as a sensor type.

A mixed electronic-ionic conductor-based bifunctional sensing layer beyond ionophores for sweat electrolyte monitoring

Noninvasive and continuous monitoring of electrolytes in biofluids based on wearable biotechnology provides extensive health-related physiological information. The state-of-the-art wearable bioelectronic ion sensors depend on the organic ionophore-based solid-contact structure of potentiometric ion-selective electrodes. This structure contains two functional sensing layers, i.e., a solid contact (ion-to-electron signal transduction) and an ionophore-containing ion-selective membrane (ISM, ion recognition). However, the potential drift, biotoxicity, and expensive organic ionophores complicate practical wearable applications. These challenges intrinsically originate from the ISM. Herein, an ISM-free wearable ion sensor based on mixed electronic-ionic conductors of tungsten bronzes is reported. These materials can serve as a bifunctional sensing layer for simultaneous ion-to-electron transduction through the redox reaction of W6+/5+ and ion recognition through crystal ion exchange. The K- and Na-adjusted WO3 disclosed Nernstian responses toward NH4+ and H+, respectively. The selectivity is comparable to or even better than organic ionophores, such as ammonia ionophore of nonactin. Further, the on-body monitoring of sweat ammonia and pH was realized using an integrated ISM-free flexible sensor. Therefore, this work offers an ISM-free concept and emphasizes the importance of developing next-generation ISM-free wearable bioelectronic ion sensors.

Influence of the Composition of Plasticizer-Free Silicone-Based Ion-Selective Membranes on Signal Stability in Aqueous and Blood Plasma Samples

Solid-contact ion-selective electrodes (SC-ISEs) in direct long-term contact with physiological samples must be biocompatible and resistant to biofouling, but most wearable SC-ISEs proposed to date contain plasticized poly(vinyl chloride) (PVC) membranes, which have poor biocompatibility. Silicones are a promising alternative to plasticized PVC because of their excellent biocompatibility, but little work has been done to study the relationship between silicone composition and ISE performance. To address this, we prepared and tested K+ SC-ISEs with colloid-imprinted mesoporous (CIM) carbon as the solid contact and three different condensation-cured silicones: a custom silicone prepared in-house (Silicone 1), a commercial silicone (Dow 3140, Silicone 2), and a commercial fluorosilicone (Dow 730, Fluorosilicone 1). SC-ISEs prepared with each of these polymers and the ionophore valinomycin and added ionic sites exhibited Nernstian responses, excellent selectivities, and signal drifts as low as 3 μV/h in 1 mM KCl solution. All ISEs maintained Nernstian response slopes and had only very slightly worsened selectivities after 41 h exposure to porcine plasma (log KK,Na values of -4.56, -4.58, and -4.49, to -4.04, -4.00, and -3.90 for Silicone 1, Silicone 2, and Fluorosilicone 1, respectively), confirming that these sensors retain the high selectivity that makes them suitable for use in physiological samples. When immersed in porcine plasma, the SC-ISEs exhibited emf drifts that were still fairly low but notably larger than when measurements were performed in pure water. Interestingly, despite the very similar structures of these matrix polymers, SC-ISEs prepared with Silicone 2 showed lower drift in porcine blood plasma (-55 μV/h, over 41 h) compared to Silicone 1 (-495 μV/h) or Fluorosilicone 1 (-297 μV/h).

A Facile Method for the Fabrication of the Microneedle Electrode and Its Application in the Enzymatic Determination of Glutamate

Herein, a simple method has been used in the fabrication of a microneedle electrode (MNE). To do this, firstly, a commercial self-dissolving microneedle patch has been used to make a hard-polydimethylsiloxane-based micro-pore mold (MPM). Then, the pores of the MPM were filled with the conductive platinum (Pt) paste and cured in an oven. Afterward, the MNE made of platinum (Pt-MNE) was characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). To prove the electrochemical applicability of the Pt-MNE, the glutamate oxidase enzyme was immobilized on the surface of the electrode, to detect glutamate, using the cyclic voltammetry (CV) and chronoamperometry (CA) methods. The obtained results demonstrated that the fabricated biosensor could detect a glutamate concentration in the range of 10-150 µM. The limits of detection (LODs) (three standard deviations of the blank/slope) were also calculated to be 0.25 µM and 0.41 µM, using CV and CA, respectively. Furthermore, the Michaelis-Menten constant (KMapp) of the biosensor was calculated to be 296.48 µM using a CA method. The proposed biosensor was finally applied, to detect the glutamate concentration in human serum samples. The presented method for the fabrication of the mold signifies a step further toward the fabrication of a microneedle electrode.

Hofmeister Effects Shine in Nanoscience

Hofmeister effects play a crucial role in nanoscience by affecting the physicochemical and biochemical processes. Thus far, numerous wonderful applications from various aspects of nanoscience have been developed based on the mechanism of Hofmeister effects, such as hydrogel/aerogel engineering, battery design, nanosynthesis, nanomotors, ion sensors, supramolecular chemistry, colloid and interface science, nanomedicine, and transport behaviors, etc. In this review, for the first time, the progress of applying Hofmeister effects is systematically introduced and summarized in nanoscience. It is aimed to provide a comprehensive guideline for future researchers to design more useful Hofmeister effects-based nanosystems.

Wearable Ion Sensors for the Detection of Sweat Ions Fabricated by Heat-Transfer Printing

Wearable ion sensors for the real-time monitoring of sweat biomarkers have recently attracted increasing research attention. Here, we fabricated a novel chloride ion sensor for real-time sweat monitoring. The printed sensor was heat-transferred onto nonwoven cloth, allowing for easy attachment to various types of clothing, including simple garments. Additionally, the cloth prevents contact between the skin and the sensor and acts as a flow path. The change in the electromotive force of the chloride ion sensor was -59.5 mTV/log CCl-. In addition, the sensor showed a good linear relationship with the concentration range of chloride ions in human sweat. Moreover, the sensor displayed a Nernst response, confirming no changes in the film composition due to heat transfer. Finally, the fabricated ion sensors were applied to the skin of a human volunteer subjected to an exercise test. In addition, a wireless transmitter was combined with the sensor to wirelessly monitor ions in sweat. The sensors showed significant responses to both sweat perspiration and exercise intensity. Thus, our research demonstrates the potential of using wearable ion sensors for the real-time monitoring of sweat biomarkers, which could significantly impact the development of personalized healthcare.

Ion-Selective Electrodes with Solid Contact Based on Composite Materials: A Review

Potentiometric sensors are the largest and most commonly used group of electrochemical sensors. Among them, ion-selective electrodes hold a prominent place. Since the end of the last century, their re-development has been observed, which is a consequence of the introduction of solid contact constructions, i.e., electrodes without an internal electrolyte solution. Research carried out in the field of potentiometric sensors primarily focuses on developing new variants of solid contact in order to obtain devices with better analytical parameters, and at the same time cheaper and easier to use, which has been made possible thanks to the achievements of material engineering. This paper presents an overview of new materials used as a solid contact in ion-selective electrodes over the past several years. These are primarily composite and hybrid materials that are a combination of carbon nanomaterials and polymers, as well as those obtained from carbon and polymer nanomaterials in combination with others, such as metal nanoparticles, metal oxides, ionic liquids and many others. Composite materials often have better mechanical, thermal, electrical, optical and chemical properties than the original components. With regard to their use in the construction of ion-selective electrodes, it is particularly important to increase the capacitance and surface area of the material, which makes them more effective in the process of charge transfer between the polymer membrane and the substrate material. This allows to obtain sensors with better analytical and operational parameters. Brief characteristics of electrodes with solid contact, their advantages and disadvantages, as well as research methods used to assess their parameters and analytical usefulness were presented. The work was divided into chapters according to the type of composite material, while the data in the table were arranged according to the type of ion. Selected basic analytical parameters of the obtained electrodes have been collected and summarized in order to better illustrate and compare the achievements that have been described till now in this field of analytical chemistry, which is potentiometry. This comprehensive review is a compendium of knowledge in the research area of functional composite materials and state-of-the-art SC-ISE construction technologies.

Detection of Interfering Ions Using Ion Flux Phenomena in Flow-Through Cl-ISEs with Ion Exchange Membranes

Ion-selective electrodes (ISEs) are among the most successful electrochemical sensors used in various applications because of their ability to measure electrolyte concentrations in liquids easily. It is common practice to suppress ion fluxes through the ion-sensitive membranes in ISEs because such fluxes worsen the lower limit of detection. In this study, we propose a method to detect interfering ions using this ion flux phenomenon. As a proof of principle, a flow-type Cl-ISE based on an ion exchange membrane loaded with the target ion chloride was used to acquire transient potential profiles during standstill after the introduction of liquids containing various ion species. When the target ion of the ion-sensitive membrane was measured, there was almost no change in potential over time. In contrast, when hydrophilic interfering ions were measured, the potential gradually decreased, and when hydrophobic interfering ions were measured, the potential gradually increased. The direction and intensity of these changes over time depended on the ion species and concentrations. The main reason for these potential changes is presumed to be the change in the local ionic composition of the sample near the sensing membrane due to ion exchange between the sample and membrane. This phenomenon could not be observed in a hydrophobic ion exchanger membrane doped with a quaternary ammonium salt and was characteristically observable using hydrophilic ion exchange membranes with a high charge density and a high ion diffusion rate. Finally, using a high-throughput flow-type system, we demonstrated the detection of interfering ions in solutions containing multiple ion species by using the ion flux phenomenon.